1. Preheat the oven to 325 degrees F. Generously butter a 13x9-inch baking dish with the butter. Set it in a larger baking pan. Set a kettle of water on to heat.

2. Spread the rice and raisins, if using, in the buttered baking dish. Pour the milk into a large saucepan; add the vanilla bean, if using. Heat the milk until small bubbles appear around the edges. Remove from the heat. If you used the vanilla bean, let steep, covered, for 10 minutes.

3. Beat the eggs and sugar in a large bowl until foamy. If using the vanilla bean, remove it from the scalded milk; carefully split the bean and scrape the soft center into the milk. Discard the pod. Gradually whisk the milk into the beaten eggs until blended. Add the lemon zest and the vanilla extract, if using.

4. Pour half the custard mixture over the raisins and rice in the baking dish; stir to distribute evenly. Place the larger baking pan on the center oven rack; pour remaining custard mixture into baking dish. Sprinkle the top evenly with nutmeg.

5. Carefully pour enough very hot water into the larger baking pan to come halfway up the sides of the baking dish. Bake for 1 hour and 15 minutes, or until the custard is almost set and the edges are golden. Let the pudding cool in the water bath. Serve at room temperature or chilled, cut into squares.